APPARATUS FOR DETERMINING WITH HIGH RESOLUTION THE POSITION OF EDGES OF A WEB

Apparatus for scanning a document includes an array of light emitting diodes, or optical fibers, arranged across the width of the document and extending further than its edges, a corresponding array of photo-diodes, or fiber optics connected to photo-diodes, for providing analog intensity signals indicative of the degree of transmission of light to each sensor, and an analyzing circuit responsive to each of the intensity signals to determine the condition of the document, and especially to determine the position of the edges of the document, and the size of any pin-holes or tears in the document. According to the invention, the analyzing circuit derives digital signals indicative of the relative intensity value of each analog signal with regard to three or more predetermined threshold values of intensity, each threshold level corresponding to a different degree of obscuration of a sensor by the corresponding area of the document. If a threshold value T3 is exceeded, but a higher threshold value T4 is not exceeded, then the circuit assumes that the corresponding sensor indicates 1/4 obscuration, and that for example the edge of the document lies at a distance from the neighboring sensor equal to 1/4 of the separation between adjacent sensors.